76 OAK. [CH. V 



Transverse Section. 



The pith cells contain living protoplasm and also 

 starch at certain times of the year, their walls are thick- 

 ened and pitted. The pith presents an irregular outline 

 because the primary vascular bundles project into it all 

 round in the form of blunt wedges. At the ends of these 

 wedges, or apparently embedded in the pith, are the first 

 formed vessels, of narrow diameter and lined with a spiral 

 thickening. Spiral vessels occur nowhere else in the 

 wood 1 . The most obvious elements in the transverse 

 section are xylem vessels, looking like holes of various 

 sizes punched in the section, the largest being in 

 tangential lines in the parts of the xylem formed in the 

 spring. The medullary rays are seen running in radial 

 lines across the section, some of them one cell in thickness, 

 others consisting of many cells, and containing in certain 

 seasons large quantities of starch. The way in which the 

 rays bend round the larger vessels should be noted ; this 

 distortion is due to the great increase in size of those 

 cambium cells which turn into vessels, pushing the rays 

 out of their true radial course. 



Besides the vessels there are a large number of 

 thick-walled woody elements which make up the rest 

 of the xylem. These are not easy to classify as seen 

 in transverse section, the wood-parenchyma may how- 

 ever be often distinguished by the starch grains which 

 it contains. 



1 It must be remembered that the spiral character is not perceptible 

 in transverse sections. 



